The boy was robbed at Rotherham Leisure Centre in Eastwood, Rotherham, last Thursday, between 5.40pm and 6.20pm.
His attacker, who was wearing a leather jacket over a hooded top, is also a boy, believed to be around the same age.
Police said the 11-year-old victim was reportedly waiting in the entrance of the leisure centre when he was approached by the other boy, who began talking to him and asked him for money.
As the victim began walking away, the attacker continued talking to him before hitting him in the face and taking a small sum of cash.
Police said the young victim sustained minor injuries and was left 'very upset'.
PC Rebecca Rushton, who is investigating, said: "This was an unprovoked attack on a young boy by someone of a similar age and we have been making enquiries to understand exactly what has happened.
"I'm sure any parent would hate for this to have happened to their child and I would urge anyone who may have been in the leisure centre around the time of the incident to please report it."
Anyone with information is asked to call police on 101, quoting incident number 825 of November 30.